1. Field of the Invention
The present invention relates to an image recording material having a radiation sensitivity, more specifically, to an image recording material suitably used as a negative planographic printing plate precursor with which it is possible to perform a so-called direct plate making for directly making a plate from a digital signal from a computer or the like by using a laser.
2. Description of the Related Art
As a conventionally known method of forming an image with a photopolymerizable composition by light exposure, there are various kinds of known methods such as a method of forming a hardened relief image by forming a recording layer using a photopolymerizable composition containing an ethylenically unsaturated compound and a photopolymerizable initiator on the surface of a support and then subjecting it to imagewise exposure to polymerize and cure the ethylenically unsaturated compound in a light-exposed portion, followed by removing a light-unexposed portion by dissolution; a method of forming an image by changing the bonding strength of a photopolymerizable composition layer (recording layer) to a support by light exposure and then removing the support; and a method of forming an image by utilizing a change in the adhesion of a toner to a photopolymerizable composition caused by light. The photopolymerization initiator used in each of these methods is an initiator that is responsive to light centered in the ultraviolet region having a wavelength of 400 nm or less, such as benzoin, benzoin alkyl ether, benzyl ketal, benzophenone, anthraquinone, benzyl ketone or Michler's ketone.
Along with recent developments in image formation technologies, there has been a strong demand for a photosensitive material exhibiting a high sensitivity to light in the visible region, and a large number of photopolymerizable compositions have been proposed, for example, a composition having a sensitivity range of about 500 nm for a laser plate making method using a 488 nm-oscillation beam of an argon ion laser. Further, research is being actively conducted into a photopolymerizable composition for light having long wavelength regions exceeding 600 nm for a laser print making method using a He—Ne laser and a semiconductor laser and for full-color image reproduction technology.
As the above photopolymerizable composition, known techniques include utilizing an ethylenic unsaturated compound and a photopolymerization initiator system having a cyanine dye whose heterocycles are connected by a mono-, tri-, penta-, or hepta-methine chain, and having a specific structure, and an s-triadine derivative having a specific structure (see Patent Publications 1 and 2, for example). Also, a photopolymerizable composition has been proposed wherein a polymerization initiator system is formed of a squarylium compound having a specific structure and a specific s-triadine compound (see Patent Publication 3, for example).
However, regarding the active radical generation capability of general photopolymerizable initiators, it is known that sensitivity is rapidly diminished in relation to light having a wavelength of 500 nm or more, and particularly to light having a wavelength of 600 nm or more, due to a reduction in light excitation energy, and the above photopolymerizable compositions proposed so far do not have a satisfactory level of sensitivity to light having such a large wavelength. Further, since the conventional photopolymerizable compositions have a problem that the photopolymerization reaction proceeds when used under white fluorescent lighting, it is difficult to obtain a photopolymerizable composition having good stability at present.
In order to solve the problems relating to low sensitivity and handling difficulties under white light in the above-described photopolymerizable compositions, a photopolymerizable composition and the like containing an ethylenic unsaturated compound, a specific dye, and a photopolymerization initiator (triadine compound or the like) have been proposed (see Patent Publication 4, for example).
However, a planographic printing plate using such a composition for its recording layer has a problem that a reduction in recording sensitivity is caused by the deactivation of radicals generated from the photopolymerization initiator due to influence of oxygen in the air, and it is necessary to form an oxygen shielding layer made from polyvinyl alcohol and the like on an image recording layer in order to ensure sufficient sensitivity. In a planographic printing plate having an oxygen shielding layer on a surface of the image recording layer, a water washing process for eliminating the oxygen shielding layer, which is water-soluble, has been typically conducted before development for the purposes of reducing development time and preventing deterioration of stability during development.
Though it is possible to ensure good developability by performing such a water washing process, there has been a demand for a method for ensuring good developability without performing the water washing process since the water washing process requires a device and a waste water treatment therefor. In order to improve the oxygen shielding layer elimination capability (developability) as well as to eliminate the water washing process, a method of adding a hydrophilic compound which is more hydrophilic than polyvinyl alcohol to the oxygen shielding layer has been considered. However, though developability is improved by such a method, there are concerns that the method causes a reduction in sensitivity, and the method has problems such as plate life is reduced when exposed with the same amount of light as the above method and the like.    [Patent Publication 1] JP-A-58-29803    [Patent Publication 2] JP-A-4-31863    [Patent Publication 3] JP-A-4-106548    [Patent Publication 4] JP-A-2000-131837